move get_android_asset_manager to header file

engine/os/android/APKFile.cpp

@@ -33,8 +33,6 @@ OTHER DEALINGS IN THE SOFTWARE.
 namespace crown
 {
 
-AAssetManager* get_android_asset_manager();
-
 static AAssetManager*	g_android_asset_manager = NULL;
 
 //-----------------------------------------------------------------------------

engine/os/android/APKFile.h

@@ -35,6 +35,8 @@ OTHER DEALINGS IN THE SOFTWARE.
 namespace crown
 {
 
+AAssetManager* get_android_asset_manager();
+
 /// Android assets wrapper
 class APKFile
 {